I could not compile your code it gives an errors, I've already got the libraries installed. I'm trying to read through your code to try to understand why it will not compile.
../main.ino:38:1: error: 'MCP3421' does not name a type
MCP3421 MCP = MCP3421();
^
../main.ino: In function 'void setup()':
../main.ino:65:3: error: 'MCP' was not declared in this scope
MCP.init(0x68,3,3);
^
../main.ino: In function 'void loop()':
../main.ino:70:9: error: 'MCP' was not declared in this scope
while(MCP.ready()==0);
^
../main.ino:74:6: error: 'MCP' was not declared in this scope
l1=MCP.getLong();
^
../main.ino: In function 'void sprintDouble(char*, double, byte)':
../main.ino:110:17: warning: zero-length gnu_printf format string [-Wformat-zero-length]
sprintf(str,"");
^
../main.ino:129:36: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
while( frac1 = frac1 / 10 & cnt-- )
^
../main.ino:108:8: warning: unused variable 'sgn' [-Wunused-variable]
byte sgn=0;
^
make: *** [main.o] Error 1
Error code 2
I did run it with the hex file and this is the output
Voltage @ pin A1 1999906
Voltage @ pin A2 1499984
Voltage @ pin A3 999984
Voltage @ pin A4 700000
Voltage @ pin A1 700000
Voltage @ pin A2 1499984
Voltage @ pin A3 1499984
Voltage @ pin A4 700000
Voltage @ pin A1 700000
Voltage @ pin A2 1499984
Voltage @ pin A3 1499984
Voltage @ pin A4 700000
Voltage @ pin A1 700000
Voltage @ pin A2 1499984
Voltage @ pin A3 1499984
Voltage @ pin A4 700000
Voltage @ pin A1 700000
Voltage @ pin A2 1499984
Voltage @ pin A3 1499984
Voltage @ pin A4 700000
Voltage @ pin A1 700000
Voltage @ pin A2 1499984
Voltage @ pin A3 1499984
Voltage @ pin A4 700000
Voltage @ pin A1 700000
Even tried to compile in Arduino IDE but same errors ??
I use this in my code so every time I need to print something to the serial for debugging so when it's working you could leave it in and rather than keep highlighting out each Serial print command you just highlight out one line and nothing get's printed.
#define ENABLE_SERIAL //Add // to disable and remove for debugging at the top of your code.
then add this to the setup code so you can see that the baud rate is correct and working
#ifdef ENABLE_SERIAL
Serial.begin(57600); //which ever baud rate you need or want
Serial.println("");
Serial.println("=====================================");
Serial.println("I'M WORKING");
Serial.println("=====================================");
#endif //#ifdef ENABLE_SERIAL
Then in your loop anything you want to print to the serial port you jys add your code example below
#ifdef ENABLE_SERIAL
Serial.print("VOLTS 1"); // so you know what your readig
Serial.println("Volt_reading1,2 "); //display the incoming voltage
#endif //#ifdef ENABLE_SERIAL
So then you just remove or add // to enable or disable with one line of code.
I've attached the start up project that I first used for mine, But the only difference is this in real hardware I used an MCP3426 which is the 16bit 2 channel version and this is only because ISIS does not have the sim model for it. The readings in ISIS take a bit to update but real time they work quite well, There may be better ways of setting it up to read faster but I'm just learning how to use these devices as I've always used the ADS1115 where you can find good examples to get them working great. I did see a few for the 18bit MCP3424 but everyone set's them up differently to read and becomes confusing. I am thinking of getting one of these ordered to play with as there easy to solder than the ADS1115 at home I do have a couple of the ADS1115 breakout boards what I use for prototyping but don't like to use them in the final design hence why I used the MCP3426 bad boy